Flow controllers are typically used to control the flow rate of a fluid in a process. Flow controllers are typically comprised of a flow sensor, a flow control valve and electronics (with optional software) to control the valve in response to the flow rate detected by the flow sensor. The flow sensor may be a thermal mass flow sensor, a Coriolis mass flow sensor, a positive displacement flow meter, or the like. Some flow controllers use valves that may not completely stop the flow of fluid when the valve is in the fully closed position. These types of flow controllers have a specified amount of fluid which may leak through the controller when the flow control valve is in the “off” or fully closed position. The amount of fluid leaking through the valve under these conditions is called leakby and is defined as the amount of fluid leaking through the flow controller when the controller is in the “off” or fully closed position. Manufacturers that use flow controllers may tune their process around this leakby specification. If the flow controller degrades or fails and the leakby value increases above the specification, manufactures may incur significant production scrap. Testing a flow controller for leakby currently requires removing or isolating the device from the process equipment and measuring the leakby value using an off-line piece of measurement equipment. Shutting down the process equipment to isolate the flow controller and attach the off-line measuring equipment may cause considerable downtime for the process equipment. In capital intensive facilities such as semiconductor factories, downtime can be very expensive.